If you're ever stumped for ideas on what to write, there are two things I recommend.

A story that I recently finished was inspired by a random thought I had while walking the dogs. They were sniffing intently at a bush, and I thought to myself, "I wonder what it's like to smell things as well as they do?" My brain then followed that rabbit down a hole and a story began forming. (Incidentally, this story is now on Wattpad! Look for My Mother Runs With Wolves on my profile. Website users can click on the External Link.)

Encourage your brain to think random thoughts. In any direction.

What would it be like to live on the moon? How high could you jump?

Do alligators really survive in the sewer? What do they live on?

How much time did Richard Adams spend staring at rabbits when he wrote Watership Down? Did his friends think he was crazy?

If teleporters ever get invented, I wonder if it'll get sabotaged by the airline and automobile industries?

Encouraging your mind to wander to pointless places sometimes sparks an idea for a story. My brain is always doing this on its own (I occasionally document these random thoughts in "My Hamster Wheel Brain" blog) and I'm often accused of spacing out. Some of these random thoughts give me ideas, and I'll scribble these ideas into a file for later reference.

Wandering thoughts usually aren't enough to come up with a story plot. The way you flesh out an idea is by asking what ifs. I'll take my previous wandering examples and apply what ifs to them...

What if they started colonizing the moon, and I volunteered? Would any of my friends and family go with me? What if I was in a relationship? What would happen to that?

What if these sewer alligators were actually robots created by a corporation to build something secret underground?

What if Richard Adams made his story about cats instead of rabbits? What might have changed?

What if teleporters do get invented, but big-money lobbies get laws to be passed that make them illegal? What if a small group of scientists keeps working on one anyway, what would they do with it? Would a black market teleportation industry spring up?

Now these wandering thoughts have more meat on them, more potential to turn into stories.

If your brain doesn't wander like mine does, there are other ways. The What If Game can be powerful. Take an existing fairy tale. Then what-if the heck out of it. Here's an example: Peter Pan.

What if Peter Pan was actually a girl? She could be Petra Pan. With Lost Girls instead of Lost Boys.

What if this was set in space? Maybe Peter Pan used a jet pack instead of fairy dust. Would Tinkerbell be an alien?

What if Captain Hook was Peter's father?

What if Peter Pan was like Mad Max, and the Lost Boys ran Thunderdome?

What if Peter somehow got old? (This has already been done, in that Robin Williams movie, Hook.)

What if Tinkerbell was big and ugly like a troll, instead of tiny, fluttery and cute?

As you can see, there are countless directions you can go in! Some of them may have been done already, but there are so many possibilities, that chances are good you'll find something original.

Maintain a file or notebook for all your ideas. No matter how dumb or tiny the idea might seem, write it down. You don't have to flesh it out. Just record what you have. Twice now, I've thought of nothing but a title. No clue what the plot was, or who the characters might be. Just a story title I thought was cool. I wrote them down and set it aside for several months.

Then later, as I recorded other ideas, I would peruse my previous ideas just for fun. Often these previous ideas will stir something in my creative space, and something will take root. Months later, I now have a complete short story, called "Sell Fish", and an in-progress novel from those two disembodied story titles I once wrote down.